1. Field of the Invention
This invention relates mainly to an ultrasonic wave probe for measuring blood flow velocity, and particularly to an ultrasonic wave probe which is suitable for use in an ultrasonic blood flow volume measuring device adapted to measure blood flow by means of an ultrasonic wave.
2. Description of the Prior Art
Prior to the description of the ultrasonic wave probe of this invention, the known blood flow volume measuring device by using an ultrasonic wave will first be mentioned.
Clear information on blood vessel mechanical properties of a carotid artery serves to reveal cerebral circulation characteristics (cerebral blood vessel characteristics) which effectively predict the cerebral blood vessel trouble such as cerebral arteriosclerosis. In other words, a mimic electric circuit model of the carotid artery system is arranged to have its input impedance characteristics approximate to the hydrodynamic input impedance characteristics obtained from the blood pressure and blood flow velocity of the carotid artery, and the cerebral circulation characteristics are measured using the parameters of the thus arranged electric circuit model.
The above electric circuit model is adapted to use the modified Windkessel type model. Therefore, a blood vessel diameter, carotid artery pulse wave, blood flow velocity (absolute flow velocity), etc. are first given to obtain a blood vessel resistance R, a blood vessel inertia L and a cerebral blood vessel capacitance C which are necessary for the modified Windkessel type model. Then, the input impedance characteristics of the blood vessel are obtained from the above constants with the result that the blood vessel mechanical properties can be made clear. Accordingly, in order to study the cerebral circulation characteristics, it is required to measure the blood pressure and blood flow amount of the carotid artery before revealing the aforesaid blood vessel diameter, carotid artery pulse wave and the like.
The blood pressure may be measured by a well-known strain gauge type blood-pressure measuring device. Meanwhile, the blood flow amount can be measured in a non-watching or non-invasive manner by using an ultrasonic wave. This device is referred to herein as an ultrasonic wave type blood flow amount measuring device.
As shown in FIG. 1, in this ultrasonic wave type blood flow amount measuring device, an ultrasonic wave from a probe (transducer) 3 is applied through the body surface to a carotid artery 1 and a reflected wave (a doppler signal) of an ultrasonic wave from the carotid artery 1, that is, from the blood flow in the blood vessel 1, is used to measure a blood flow velocity and blood flow amount or to display its waveform. In this case, however, measured values are greatly different as indicated by the following table according to an ultrasonic wave incident angle .theta..sub.T with respect to the carotid artery 1.
______________________________________ Incident Angle Blood Flow Velocity Doppler Frequency .theta..sub.T (.degree.) (cm/sec) (Hz) ______________________________________ 80 20.8 694 70 41.0 1368 60 60.0 2000 50 77.1 2571 40 91.9 3064 ______________________________________
The above values are measured by using the blood of a human being having blood flow velocity of 60 cm/sec and by causing the ultrasonic wave probe 3 to touch the body surface where the blood flows through the blood vessel 1 in a direction of arrow a as shown in FIG. 1.
As seen from the above table, the blood flow velocity with the smallest error or no error is obtained when .theta..sub.T is 60.degree., but error is increased according as .theta..sub.T deviates from 60.degree.. In this example, when the incident angle .theta..sub.T is deviated .+-.10.degree. from 60.degree., the measured values are changed about 25 to 30%. In other words, correct results cannot be obtained unless the ultrasonic wave incident angle .theta..sub.T is always held at 60.degree..
However, when handling the probe 3 in a practical case, a physician, for example, must manually hold the probe 3 to indirectly touch the surface adjacent the carotid artery 1 of a patient. Since manual manipulation is unsteady and easily variable under clinical conditions the incident angle .theta..sub.T becomes irregular case by case. For this reason, the blood flow velocity obtained by using the known probe 3 will be different at every measurement. As a result, the measurement accuracy is quite low.
Further, in the prior art, the ultrasonic wave probe for measuring the blood flow velocity and that for measuring the blood vessel diameter deviation are constructed separately from each other. Therefore, the diameter deviation and the blood flow velocity cannot be simultaneously measured at the same place of the blood vessel with the result that the measurement accuracy thereof is lowered.